Direct fired heaters find wide application, particularly in oil refineries, where they are used for the purpose of preheating petroleum or petroleum derived feed-stocks for further processing to produce such products as fuel gas, gasoline, diesel fuel, heavy fuel oil and coke. The feed-stocks are of variable composition and boiling range and require that they be preheated to varying temperatures for further processing. Some of the applications considered would be as follows:
Delayed Coking Heater Service, which is the primary focus of the subject invention, and which involves preheating of high boiling point feed-stocks, such as residua, to high temperature, and transferring the heater effluent to a coke drum where it is held for a period of time, sufficient, to convert the feed-stock charged to a product slate consisting of fuel gas, low boiling point liquids, high boiling point liquids and coke.
Direct fired heaters in this service operate at the severest conditions of any in common oil refinery service, with the exception of direct fired heaters in thermal cracking service. Thus the design strategies applicable to heaters in delayed coking service should, in principle, be applicable to other services as well, these to include:
Crude Heater Service, wherein pretreated as-received crude is pre-heated to high, but somewhat lower temperature than that used in delayed coking, prior to being introduced to an atmospheric distillation column, where a large spectrum of products are separated from one another, to yield such end products as refinery gas, gasoline, diesel fuel, heavy fuel oil, and very high boiling point residua.
Vacuum Heater Service, wherein residual feed-stocks from atmospheric distillation are preheated to high temperature, under vacuum, followed by processing in a vacuum distillation column, to separate such products from the heater effluent as gas, liquids with a wide range of boiling points and a very high boiling residuum suitable for use as a delayed coking feedstock.
Cracking Heater Service, wherein a low molecular weight hydrocarbon gases or moderate molecular weight vaporized hydrocarbon liquids, in the presence of steam, is converted to a gaseous product containing unsaturated hydrocarbon gases, such as ethylene, propylene, and other products consisting of higher molecular weight hydrocarbons. Optimum conversion is obtained by heating the steam-hydrocarbon mixtures to elevated temperature in a short residence time conventional heater, designed to provide a residence time of less than one second. Comparable operating conditions are obtainable in a heater designed in accordance with the subject invention and comprise another focus of the invention.
Conventional direct fired heaters used for the above services are usually provided with two sections, a radiant section and a convection section. The radiant section consists of a refractory lined enclosure having one or more tubular heating coils, thru which the process fluid flows. The heating coils surround a grouping of one or more burners fueled by gas, oil or other combustible. The heating coils are arranged to form a combustion chamber into which high temperature combustion products, generated by the burners, are discharged. Heat is transferred from the combustion products to the heating coils and contained process fluid, principally by radiation.
Process fluid, and/or fluid preheating for other services is usually conducted in the convection section, prior to further post-heating of the convection section streams in the radiant section or elsewhere. The convection section consists of a refractory lined enclosure containing multiple rows of closely spaced tubes, the spaces between tubes, forming multiple channels thru which flow relatively low temperature combustion products, exiting the radiant section. The combination of high velocity combustion gas flow, the low temperature of the combustion products and the relatively small radiating volume of the combustion gases result in predominately convective transfer of heat from combustion products to process fluid.
Because of the high temperature to which hydrocarbon fluids processed in the radiant section are subjected, fluids at the inside wall of the tubular radiant section heating elements experience thermal decomposition, which results in internal coke deposition, the thickness of the deposits being greatest at locations where tube metal temperatures are highest. These deposits restrict the flow of heat from the tube wall to the contained process fluid so that the tube wall temperature eventually reaches design temperature. At this point, referred to as end of run conditions, the heater must be shut down or on-stream decoking procedures initiated, to avoid tube damage. The time interval between decoking procedures is referred to as run length. Since decoking involves use of additional labor and, utilities and incurs costs due to lost production, means of eliminating or minimizing the frequency of decoking is a worthwhile pursuit.
In the case of the subject invention, used in cracking heater service, extended run lengths are obtainable without the need for heater shut down for periodic removal of tubular coke deposits, as in the case of conventional heaters. Coke instead deposits on the surfaces of the particulates used as a heat transfer medium. Such deposits are removable while the heater is in operation by adding appropriate quantities of air to the burner flue gas effluent in the vessel used for particulate preheating prior to transferring the particulates to the vessel used for feed preheating and cracking.